220 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
practice, will also enable one to determine with similar ease the 
amount and kind of double refraction and the degree of dispersion. 
As will be seen from the diagram (pl. 1, fig. 2), the main principle 
of the instrument is the same as that of the reflectometer, the refrac- 
tive index being measured against a standard of highly refracting 
glass by means of the angle of total reflection, which of course 
diminishes the nearer the index of the stone approaches that of the 
standard. It is, however, in the details of construction that such 
a marked advance has been made, and it is these details which make 
all the difference in practical work. To use this instrument all that 
has to be done is to place the stone under examination in optical 
contact with the flat surface of the dense glass, and arrange it so that 
a good light (preferably monochromatic) enters the instrument 
through the lower lenticular opening, when the refractive index 
is read off directly on a scale, without calculation.! 
Some little advance has also been made in the construction of the 
dichroscope for determining pleochroism. As will be seen from the 
illustration (pl. 1, fig. 3), the instrument in use to-day is provided 
with a revolving holder tipped with wax, to which the stone is readily 
fixed, leaving both hands free—a detail, but again it is such details 
that count in practice. 
Taking the properties of precious stones as a whole, the great point 
about them is the remarkable combination of qualities; it is not so 
much that they have optical properties which make them extraordi- 
narily beautiful, or that they have remarkable hardness and dura- 
bility, but they have both, and it is the impossibility of reproducing 
this combination in any other material that renders the detection of 
imitations a matter of ease in the hands of anyone familiar with the 
facts. : 
Of course, glass is the obvious material to use in the production 
of imitation gems, and, as I have indicated, it has been so used from 
time immemorial. And, in later times, while science was equipping 
the expert in precious stones with the means of identifying them 
with certainty, the maker of imitations was also invoking its aid 
in the production of more successful imitations. 
In modern times the manufacture of imitation gems on scientific 
lines was introduced by Strasser in Vienna; hence the name ‘‘strass,”’ 
although ‘‘paste” is the more commonly used term. 
The finest of such modern paste bears little relation to the clumsy 
imitations of early times; the glass is specially prepared in order to 
combine, as far as possible, the necessary optical qualities with a 
fair amount of durability. It is well known that by using lead 
1 It is impossible here to give any detailed account of the construction and use of this instrument. Full 
particulars will be found in The Herbert Smith Refractometer, published by J. H. Steward, 405 Strand, 
